A propeller blade is fixed in a propeller hub by the blade shank, which has means for anchoring in the hub according to a longitudinal axis of the blade, and therefore according to a radial axis of the hub. The blade shank often comprises an anchoring bush, which provides a mechanical linkage between an anchoring in the hub and a fixing to the aerodynamic portion of the blade. Accordingly, there is known FR 663270, which discloses a blade the shank of which is fixed by screwing in an anchoring bush, which is itself anchored radially relative to a hub.
However, inspection of the fixing of the aerodynamic portion of the blade in the blade shank and, where applicable, in an anchoring bush is not easy. It is necessary to dismantle the blade from the hub and then dismantle at least the anchoring bush in order to check the condition of the fixing. However, the visual inspection of such an important assembly must be able to be carried out quickly and easily.
Furthermore, in the case of particularly powerful aircrafts, the propeller blades are subjected to very considerable forces, so that the assembly of the aerodynamic portion to the blade shank must be sufficiently strong to retain the blade in the hub, to transmit the movement of the hub to the blades and at the same time to transmit the forces of the blades to the hub (propulsion of the aircraft especially).
The blade shank, and more particularly the anchoring bush, and the fixing of the aerodynamic portion to the blade shank are therefore subjected to high stresses—especially during high accelerations—and to high levels of vibration. They are additionally subjected to considerable forces in a continuous and prolonged manner during long flights, and to impacts in the event of collisions of a blade with a bird, for example.
In the event of rupture of the blade shank and/or of at least part of the anchoring bush while a propeller is rotating, the blade is released, which can have serious consequences for the aircraft and its occupants, or for people and structures on the ground.
For that reason, partial solutions have been proposed, as in U.S. Pat. No. 4,685,864, which describes a blade in which a bundle of fibers extends from the blade shank, in which one of its ends is anchored, to the free end of the blade, in which its second end is anchored. The bundle of fibers is not tensioned on manufacture and prevents part of the blade from being ejected in the event of rupture of its aerodynamic portion. This solution remains partial, however, because it does not solve the problem of the rupture of the blade shank or of the anchoring bush.
Other solutions have been proposed in theory, for example by FR 2942454 and FR 2948425, in which a retaining device is anchored in the blade on the one hand and in a hub on the other hand. However, such devices, which would require adaptation of the hub of each propeller, are heavy and expensive to implement.
Furthermore, in the event of rupture of a blade, the whole of the hub must be dismantled in order to replace the damaged retaining device and blade.